int singleNumber(int A[], int n) {

        int once = 0;

	int twice = 0;

	int three = 0;

	for (int i = 0; i < n; ++i)

	{

		//在计算新的once 前，计算twice

		twice |= once & A[i];

		//计算新的once

		once ^= A[i];

		three = ~(once & twice);

		//将3次的1都清理掉

		once &=  three;

        twice &= three;

	}

	return once;

    }